The demand for the motors of automobiles, household electrical appliances, information appliances and industrial machinery is expanding, and the bearing used for the rotational part of the motors is required to have quietness in addition to durability. Reduction of the noise generating from the bearings of the motors (for example, brushless motors and fan motors) in the household electrical appliances and information appliances is a particularly urgent issue because those appliances are operated somewhere near the people.
The noise of the bearing is largely affected by the kind of grease composition enclosed in the bearing. The sliding surface of the bearing is lubricated with an oil film formed by the grease composition. Depending on the kind of grease composition, the formed oil film may be distorted and such distortion is considered to vibrate the bearing, which will cause the noise.
For the purpose of improving the noise performance of the bearings, a variety of grease compositions are proposed. For example, JP 2010-222516 A describes a grease composition designed for the rolling bearings of automotive motors, comprising a particular ester type synthetic oil as the base oil, and a lithium soap as the thickener.
However, the above-mentioned grease composition comprising the lithium soap as the thickener shows insufficient durability at high temperatures although the noise performance of the bearing can be relatively improved. Further, the above-mentioned grease composition tends to swell the sealing member (made of rubber or resin) for sealing the gap between the inner ring and the outer ring of the bearing, so that the sealing performance of the sealing member may be impaired.
JP 2004-211797 A describes a grease composition to be enclosed in the rolling bearing, comprising as the thickener a diurea compound containing a fibrous material with a fiber length of at least 3 μm. JP 2008-143979 A describes a grease composition comprising a particular synthetic hydrocarbon oil as the base oil, a diurea compound as the thickener, and a zincate or ammonium salt of organic sulfonic acid.
The grease compositions disclosed in JP 2004-211797 A and JP 2008-143979 A, which comprise the diurea compounds as the thickeners show high durability at high temperatures and improved noise performance of the bearing. However, in response to the recent strict demand for quietness, it is desired to secure the low noise performance of the bearing at high temperatures not only in the initial stage, but also for an extended period of time, that is, the life of bearing with low noise performance is desired to extend. To satisfy the above-mentioned severe requirements, those conventional grease compositions should be further improved.
In the bearings incorporated in the small-sized motors of the household electrical appliances and information appliances, there is still a high demand for improvement of the life that can secure the low noise performance under high temperature conditions.